Optical parametric oscillation is a nonlinear process that converts a single input laser beam or a pump beam source into two lower energy-beams known as the signal beam and the idler beam. The wavelengths/frequencies of the various beams λpump/fpump, λsignal/fsignal, and λidler/fidler must satisfy:
                                          1                          λ              pump                                =                                    1                              λ                signal                                      +                          1                              λ                idler                                                    ,                            (        1        )            or equivalentlyωpump=ωsignal+ωidler.  (2)Ideally, energy is conserved since the sum of photon energies of the signal beam and the idler beam is equal to the photon energy of the pump beam, that is, the energy of a photon is proportional to the frequency thereof. Therefore, it is possible to implement a laser capable of being continuously tuned over a wide range of wavelengths by adjustment of the optical parametric oscillation only. The optical parametric oscillation can be tuned to create a multicolor laser system by changing the grating spacing of the nonlinear crystal, for example, which can be achieved by controlling temperature of the nonlinear crystal, or accomplished by rotating the crystal relative to the incident light beam.
FIG. 1 shows a schematic setup of a typical optical parameter oscillator. As shown, a pump beam is generated from a pump laser 10 to propagate through an optically-nonlinear crystal 12 placed in an optical resonator comprised of a pair of mirrors 14. While traveling through the optically-nonlinear crystal 12, a small portion of the pump beam is converted into a signal beam and an idler beam. The signal beam and/or the idler beam are fed back by the mirrors 14I and 14O of the optical resonator. When the pump beam is coupled into the nonlinear optical crystal 12, the signal beam and the idler beam may be generated depending on the intensity of the pump beam and the reflectivities of the mirrors 14. Each optical parametric oscillator has a characteristic pump-intensity threshold. At and above the threshold, the amplification of the signal and idler beams compensates the resonator roundtrip loss caused by residual mirror transmission, crystal absorption, scattering, etc. If the optical parametric oscillator is pumped above the threshold, a significant amount of pump beam is converted into signal and idler radiation. In practice, the input mirror 14I is designed with maximum reflectivity for the signal beam and idler beam, and the output mirror 14O determines whether the optical parametric oscillator is singly- or doubly-resonant. That is, the output mirror 14O determines the proportions of the signal beam and the idler beam to be fed back to the nonlinear crystal 12 and resonated in the optical resonator.
Applications of optical parametric oscillation include light detection and ranging (LIDAR), high-resolution spectroscopy, medical research, environmental monitoring, display technology and precision-frequency metrology. In coherent-detection applications of LADAR, vibrometry, and free-space optical (FSO) communication, a tunable, narrow-line, high-power source with wavelength (λ) of 1.5 microns is required. For example, coherent LADAR could require a source of about 10 Watts to about 100 Watts at a wavelength (λ) of about 1.54 microns with tunability of 1 nanometer over a 50 micro-second chirp, and linewidth as narrow as 50 kHz. It is likely that LADAR will rely on gas lasers to achieve these narrow linewidths in the near term. Similarly, airborne, free-space-optical communications will require a wavelength of about 1.5 microns with some tunability within the C-band and the linewidths of 100 kHz in a coherent-detection mode. Airborne free-space-optical communication will rely on existing telecommunication components in the near term, such as a 1 micro-Watt laser diode, followed in series by erbium-doped fiber amplifiers (EDFA's) to achieve powers of 10 Watt. Polarization-maintaining erbium-doped fiber amplifiers are expensive; moreover, high-end, erbium-doped fiber amplifiers may provide no more than tens of Watts of power each. As the airborne free-space-optical range requirements increase, it is a challenge for sources to provide more power without sacrificing linewidth.
Nonlinear optics have been applied to the above missions for a number of years. For example, pumping optical parameter oscillators (OPO's) with Nd:YAG laser sources is a highly reliable approach for tunable, high-power sources. Materials used to pump optical parameter oscillators include periodically-poled lithium niobate (LiNbO3 or PPLN). Tens of Watts at a wavelength of about 1.064 micrometers can be pumped into PPLN prior to approaching its laser-damage threshold. However, these types of optical parametric oscillators tend to have fairly broad linewidths.
Narrow-linewidth operation (Δλ˜0.02 nanometer) of optical parametric oscillator has been achieved using a Littrow configuration disclosed in literatures such as “Littrow Configuration Tunable External Cavity Diode Laser with Fixed Output Beam” by C. J. Hawthorn, K. P. Weber, R. E. Scholten in Review of Scientific Instrument, Vol. 72(12) pp4477–4479, December 2001. Bosenberg et al. have also demonstrated a single-crystal optical parametric oscillator based on KTiOPO4 (KTP), a grating, and a tuning mirror. These disclosures indicated that fine tuning of one mirror provides a wavelength-selection mechanism, in which the optical parametric oscillator can be selectively seeded for a given narrow line. However, in these optical parametric oscillators, the resonator, the grating, and the nonlinear crystal are separate devices such that precise alignment is highly demanded, but it is laborious and time consuming. Further, this conventional approach involves mechanically tuning the mirror.